1. Field of the Invention
The present invention relates to an apparatus for developing, e.g., an electrostatic latent image on a recording sheet by using a developing liquid and, more particularly, to a liquid-type developing apparatus including a developing head having a hermetic structure.
2. Description of the Related Art
Conventionally, a liquid-type developing apparatus of this type has an arrangement as follows. FIG. 19 is a schematic diagram showing the arrangement of an apparatus of this type disclosed by U.S. Pat. No. 4,289,092. This apparatus has a developing head 1, a developing liquid container 3, and a suction pump 5. The developing liquid container 3 is coupled to a developing liquid inlet port 1i of the developing head 1 through a pipe 2. The suction pump 5 is coupled to a developing liquid outlet port 1j of the developing head 1 through a pipe 4.
The developing head 1 is constituted by lower and upper base bodies 1a and 1b, as shown in the enlarged view of FIG. 20. The lower and upper base bodies 1a and 1b respectively form rod-like shapes. The lower and upper base bodies 1a and 1b are integrated by being bonded to each other. Buffer chambers 1c and 1d are formed in the developing head 1. The buffer chamber 1c communicates with the developing liquid inlet port 1i. The buffer chamber 1d communicates with the developing liquid outlet port 1j. A plurality of partitioning walls 1e and 1f are alternately formed in the lower and upper base bodies 1a and 1b, respectively. These partitioning walls 1e and 1f constitute slit-shaped opening portions 1h. The length of the slit-shaped opening portions 1h is set to be smaller than the width of a recording sheet on which development is performed. The top portions of the partitioning walls 1e and 1f, serve as support surfaces of the recording sheet. The height of the top portion of each partitioning wall 1e is set to be slightly smaller than that of the top portion of each partitioning wall 1f. Gaps formed by the differences between the heights of these top portions serve as the developing liquid feed paths. Reference numeral 1g denotes a slit for suction drying.
With the arrangement as described above, an electrostatic latent image formed on the recording sheet is developed in the following manner. That is, the recording sheet having the electrostatic latent image formed thereon is conveyed to the developing head 1 with its surface having the latent image facing downward, and is set at a position to oppose the slit-shaped opening portions 1h. In this state, the suction pump 5 is operated. Then, air in the buffer chambers 1d and 1c of the developing head 1 is discharged by suction through the pipe 4, and the recording sheet is sucked to the slit-shaped opening portions 1h. Upon suction of the recording sheet, the entire developing head 1 is set at a negative pressure, and hence the developing liquid is drawn by suction from the developing liquid container 3 through the pipe 2. The buffer chamber 1c connected to the developing liquid inlet port 1i fills with the developing liquid. The developing liquid flows to the buffer chamber 1d connected to the developing liquid outlet port 1j through a curved path formed by the partitioning walls 1e and 1f, and is circulated to the developing liquid container 3 through the pipe 4 and the suction pump 5. In the developing head 1, the developing liquid flows through a gap formed by the difference between the heights of the partitioning walls 1e and 1f, thereby performing development of the recording sheet. If the partitioning walls 1e are formed by a metal, their top portions serve as a developing electrode.
In this conventional developing head, however, the slit-shaped opening portions 1h for development form a long member in the widthwise direction of the recording sheet. Thus, when the interior of the developing head is set at the negative pressure, the slit-shaped opening portions 1h tend to bend the recording sheet by drawing it into the slits. Then, the recording sheet is flexed, and it becomes difficult to precisely maintain the gap defined between the recording sheet and the top portions of the partitioning walls 1e. When the type of the recording sheet is changed, the stiffness of the sheet is changed, and thus the gap described above is changed to vary the developing performance.
In order to prevent this drawback, the difference between the heights of the partitioning walls 1e and 1f may be decreased to maintain the gap at a small value, or the width of the top portion of each partitioning wall 1e may be increased. With this arrangement, however, the in the recording sheet caused by the negative pressure eliminates the gap to disable flow of the developing liquid, and a variation occurs in the flow of the developing liquid to disable normal development. In order to prevent this, a large tension may be applied to the recording sheet, thus preventing the recording sheet from being bent. In this case, however, the convey mechanism for the recording sheet becomes complicated and increased in size. In addition, the recording sheet is unpreferably distorted or damaged.
Published Examined Japanese Patent Application No. 1-39109 discloses another developing head. The structure of this developing head is shown in FIG. 21. A large number of small barriers 22F are disposed in the slit-shaped opening portions in the checker manner. These barriers 22F support the recording sheet and decrease a bend in the recording sheet. Accordingly, the size of the gap is maintained at a substantially constant value, and thus a degradation in developing performance is prevented. With this developing head, however, the large number of complicated barriers 22F or the like must be formed. Thus, the structure of the head becomes very complicated, and its manufacture becomes difficult. In addition, the head becomes considerably expensive.